This invention relates to apparatus and method for measuring the quantity of a liquid transferred to or from a storage tank. More particularly, the invention relates to apparatus and method for the custody transfer of crude oil from a storage tank.
An important step in the production of oil and gas is the custody transfer of the produced crude oil from the storage tank at the well site to a pipeline, railway car, or tank truck. At the present time, when a tank of oil is transferred, or "run" the transfer is measured by manually "gauging" the depth of the oil in the tank through a hatch in the top of the tank before and after the transfer is completed. Before initiating the transfer, the person responsible for documenting the transfer (the "gauger") must climb to the top of the tank, open the tank to the atmosphere, suspend a thermometer in the oil in the tank, lower a sample-taking device known as a "thief" to take a sample in the upper third of the tank, lower a thief into the tank to take a sample at the level of the outlet, determine the free water and sediment level in the tank using the outlet level sample or using a separate measurement from the top of the tank, take the opening "gauge" (measurement of the height of the crude oil in the tank) using a tape and bob lowered from a reference point on the top of the tank, read the thermometer, climb down from the tank, break the tank outlet seal, and begin pumping oil from the tank (assuming the sediment and water level is at least four inches below the outlet). The gauger uses the samples and the temperature measurement to compute the average API specific gravity, temperature, and sediment and water content. After the pumping is completed, the gauger must close and seal the tank outlet valve, climb to the top of the tank and take the closing gauge, and complete and distribute the run tickets which document the measurements taken during the run, or transfer.
The preceding description is at best a summary and oversimplification of the custody transfer procedures used at this time. The procedures are described in detail in the "Manual of Petroleum Measurement Standards Chapter 18--Custody Transfer" published by the American Petroleum Institute (API).
Despite the fact that the above-described procedure is the procedure recommended by the API and followed by the oil and gas industry, it is well known to be dangerous to the gauger and hazardous to the environment. When the gauger opens the top of the tank, he exposes himself and the environment to the fumes inside the tank. Over the years, many gaugers have died from exposure to hydrogen sulfide fumes from an open tank. The fumes may be explosive and precautions must be taken to prevent sparks from metal contact and static electricity. Environmental groups are urging the state legislatures to enact laws which would prohibit the discharge of fumes from open tanks to the atmosphere. Additionally, the tanks deteriorate with age and prolonged exposure to components of some crude oils which can weaken the tank, making it dangerous for the gauger to climb the tank. After completing the transfer, the gauger must manually complete and distribute the run ticket which documents the transfer, which creates a potential for human error, loss of the run ticket, and delays communication of the transfer to the buyer and seller.
It is also common to have two or more tanks ("lease tanks") on a leased property near the well site which are connected to a pipeline. The pipeline is typically connected to the crude oil purchaser's stock tank, which may be many miles from the lease tanks. The gauger may travel and service lease tanks at many well sites scattered over a 200 mile or more radius from the stock tank. Typically, the gauger will go to a well site, take the opening gauge of a lease tank and go through the API procedure necessary to initiate a transfer, open the tank outlet valve to the pipeline, and then travel to another well site and do the same, and so on. It may be three to five days before the gauger returns to an open tank to take the closing gauge, close, and reseal the lease tank. Therefore, it may be three to five days before the run ticket is made and distributed and it may be three to five days before the purchaser knows how much oil was run from a lease tank and whether the same amount arrived at the purchaser's stock tank.
The fact that the above-described procedure is the API and industry standard, and has been for about sixty years, attests to the long-felt need for a custody transfer apparatus and method which does not require the gauger and the environment to be exposed to the contents and fumes existing in the tank, which does not require the gauger to open the tank to the environment, which does not require the gauger to climb on the tank, and which makes an accurate, permanent record of the transfer which can be printed out at the tank as well as electronically transferred to a remote location through a communications link with a telephone system or the like. There is also a need for such an apparatus and method which will electronically communicate with a remote location, allow a tank to be run (opened and closed) remotely without the gauger going to the lease tank, create a permanent run record, and communicate the run data to the remote location at the time the run is being made. There is also a need for such an apparatus which does not require a new opening to be made in the tank, as sour gas (such as hydrogen sulfide) becomes entrapped in the walls of operating tanks and can explode if an opening is cut in the tank.
Attempts have been made to provide apparatus which improve the tank gauging procedure. For example, CTI Manufacturing, Inc. of Houston, Tex. manufactures a sonar tank gauge known as Accu-Gage.TM.. The Accu-Gage.TM. device measures the level in a tank using an ultrasonic transducer to generate an ultrasonic pulse which travels up through the liquid and reflects from the top surface back down to the ultrasonic probe. The probe is inserted into the tank near the bottom of the tank. The probe must be located away from turbulence and internal constructions and therefore cannot be positioned in or near a turbulent inlet or outlet. Consequently, the probe location cannot be used to take a composite sample or to measure the temperature of the liquid transferred as the "nonturbulent" location may not provide a representative sample of the liquid transferred. This means that the top of the tank must be manually opened to take a sample which creates the problems previously discussed. Also, the ultrasonic probe will not read an interface or level within six inches of the probe, as it has a minimum range of six inches; and therefore it cannot be used to give a closing gauge at tank outlet level (normally 12 inches) as the probe must be mounted a minimum of 12 inches above the bottom of the tank. In other words, the tank cannot be drained down to outlet level using the Accu-Gage.TM., which unnecessarily and undesirably reduces the storage and sales capacity of the tank. Further, the presence of emulsion and particulate matter in the liquid, as is common in crude oil, can affect the reliability of the readings of an ultrasonic probe. The Accu-Gage.TM. device can communicate digitally with remote computers.
Digi-Con Corporation of Marysville, Wash. manufactures a non-intrusive ultrasonic level sensor which is mounted externally on the bottom (or at the top) of a tank and which has a transceiver for communicating with a personal computer-based monitoring system. As the Accu-Gage.TM. system, the Digi-Con system does not disclose a method of sampling the tank contents without opening the top of a tank and may be unreliable if emulsion or particulate matter is present.
Neither the Accu-Gage.TM. nor the Digi-Con system discloses a method of automatically creating a permanent run record, compensating for sediment and water content, of indicating when water is present at an unacceptable level in the bottom of the tank, of reliably measuring level when emulsion or particulate matter is present in the liquid, or of initiating, measuring, and terminating the transfer of liquid to or from a storage or lease tank from a remote location without a gauger going to the tank to open or close the tank. The shortcomings of the prior attempts to improve the tank gauging procedure are evidenced by the fact that they have had no effect on the API standard.